This invention relates to a screen for a rear projection type television set in which an image is projected onto a screen from a projection tube behind the screen and the image projected onto the screen is observed from the front of the screen.
In a projection television set of this type, the projection image is diffused by the screen to provide a certain visual field angle when viewed from the front of the screen so that a substantially bright image can be observed even from an oblique angle.
FIG. 8 shows the fundamental arrangement of the above-described rear projection type television set. In FIG. 8, reference numeral 3 designates a projector that may comprise a CRT projection tube and a projection lens, reference numeral 4 designates a screen made up of a Fresnel sheet 41 disposed adjacent to the projector 3, and reference numeral 42 designates a lenticular sheet (generally called "a lenticular lens") disposed adjacent to the emergent surface of the Fresnel sheet 41.
A Fresnel lens 41a is formed on the emergent surface of the Fresnel sheet 41, and a number of lenticulars 42a are formed on the incident surface of the lenticular sheet 42 arranged at predetermined intervals along the horizontal direction of the screen (i.e., in a direction perpendicular to the surface of the drawing). The projection light from the projector 3 emerges onto the front surface of the screen 4 passing through the Fresnel sheet 41 and the lenticular sheet 42.
In this configuration, the projection light from the projector 3 is converged into parallel rays by the Fresnel lens 41a formed on the emergent surface of the Fresnel sheet 41, and the parallel rays are diffused horizontally by the lenticulars 42a formed on the incident surface of the lenticular sheet 42, so that the directional angle of the emergent light from the lenticular sheet 42 is increased in a horizontal direction. Further, a diffusing agent is mixed in the Fresnel sheet 41 and/or the lenticular sheet 42 to diffuse the projection light in the vertical direction. Thus, the emergent light from the screen 4 has a directional angle in both the horizontal and vertical directions which increases the visual field angle in both the horizontal and vertical directions to some extent when viewed from the front of the screen. In practice, the ordinary visual field angle is .+-.40.degree. and .+-.5.degree. in the horizontal and vertical direction, respectively, depending upon the position of observation of the projection television set.
The reason why the visual field angle is large in the horizontal direction and small in the vertical direction is that the vertical diffusion is effected by mixing the diffusing agent. Specifically, if the quantity of diffusing agent is increased, then the transmittance is decreased, as a result, the contrast is lowered and the projected image becomes foggy; that is, the resolving power is reduced. Therefore, the quantity of diffusing agent to be mixed is limited. This limitation limits the increase of the directional angle of diffusion in the vertical direction.
The diffusion characteristic of a diffusing agent is isotropical. Therefore, the diffusion action is effected in the horizontal direction, in which the diffusion is sufficiently carried out with the aid of the lenticular sheet. Thus, for the entire screen, the diffusion efficiency is low.
If the quantity of diffusing agent is increased, then the output of the CRT projection tube may be increased in order to prevent the aforementioned lowering. However, this method is disadvantageous in that the service life (or durability) of the CRT projection tube is decreased and output loss is increased because, as described above, the diffusion is effected in an unwanted direction due to the diffusion characteristic of the diffusing agent.
If, on the other hand, the quantity of diffusing agent to be mixed is decreased, then the visual field angle in the vertical direction is decreased and the diffusion of light in the Fresnel sheet 41 is decreased. Therefore, the projection light, which has been partially reflected by the emergent surface of the Fresnel lens 41a, is reflected by the incident surface of the Fresnel lens 41a which forms a ghost image of the projection image on the screen 4.